The concentration of ethanol from aqueous solutions has been accomplished by distillation for many years in the production of alcoholic beverages, solvents and a variety of chemicals. Ethanol from such distillations, either alone or in combination with hydrocarbon fuels such as gasoline, has been more recently used as an automotive fuel. Contamination by water is an undesired consequence of most simple fractional distillation schemes when the alcohol is to be used as a fuel, while contamination with toxic materials is an undesired consequence when the alcohol is to be used for human consumption. In both processes, the high energy requirements for the distillation, coupled with the inherent limitation imposed by the ethanol-water azeotrope have caused various methods other than simple distillation to be investigated.
Extraction of aqueous ethanol solutions by an organic solvent has been proposed, with the extract containing both ethanol and solvent then generally being distilled to separate the product ethanol and a recycle solvent. An article by J. W. Roddy, entitled "Distribution of Ethanol-Water Mixtures to Organic Liquids" in Ind. Eng. Chem. Process Des. Dev. , volume 20, pp 104-108 (1981) indicates that a wide variety of organic solvents have been used for such extractions, but that the number of solvents having distribution coefficients greater than 0.5 for ethanol and separation factors greater than 10 from aqueous solutions (as defined in the Roddy article) are quite limited. The article indicates the general order of extraction for ethanol to be hydrocarbon=halocarbon&lt;ether&lt;ketone&lt;amine&lt;ester&lt;alcohol=phosphate. The best candidate identified in the article were 2-ethyl-1-butanol, having a distribution coefficient of 0.69 for ethanol and a separation factor of 30. The next best candidate, triisobutyl phosphate, had a distribution factor for ehtanol of 0.65 and a separation factor of 10.
It has also been proposed to conduct a distillation to separate ethanol from water with an additional solvent being added to the system so as to either enhance the separation and purity of ethanol as the overhead and of water as the bottoms, or to reverse the volatilities for ethanol and water, causing water to be removed as top product, and ethanol mixed with solvent to be removed as bottom product. Examples of such suggestions are contained in U.S. Pat. No. 2,591,672 (with a hydrocarbon as the extractive distillation solvent) and and article by C. Black entitled "Distillation Modeling of Ethanol Recovery and Dehydration Processes for Ethanol and Gasohol", in Chem. Eng. Prog., September 1980, pp 78-85, especially at pp 82-84.
While the above references indicate the desirability of extraction and extractive distillation schemes to recover ethanol from aqueous solutions, there is still a need for solvents suitable for such extraction and extractive distillation.